You succeeded in picking out a minor technical inaccuracy, but not in addressing the point the author was making about rendering performance.
It does, but unfortunately neither Webkit nor Gecko respect this. Take a PNG with an alpha channel and draw it to a canvas over a white div then read back the pixels; you get the pixels premultiplied by the white behind it. Same with other backing colors. There are cases where it won't premultiply, but unfortunately they're a minority.
(The reason I ran across this particular case is that I use PNGs to compress data, and using the alpha channel to store data is impossible because of the premultiplication.)